Betulinol is one of the more plentiful triterpenes, constituting up to 24 per cent of the outer bark of the white birch (Betula alba) and as much as 35 per cent of the outer bark and about 5 per cent of the inner bark of the Manchurian white birch (Betula platyphylla) (Hirota et al., J.S.C.I. Japan, 47:922 (1944)). It also occurs in the free state in the barks of the following trees: the yellow and black birch (Steiner, Mikrochemie, Molisch-Festschrift, p. 405 (1936)), Corylus avellana, Carpinus betulus (Feinberg et al., Monatsh, 44:261 (1924); Brunner et al., Monatsh, 63:368 (1934); and Brunner et al., Monatsh, 64:21 (1934)), and Lophopetalum toxicum (Dieterle et al., Arch. Pharm., 271:264 (1933)). The exudate from the bark of Trochodendron aralioides, which constitutes Japanese bird-lime, contains betulin palmitate (Shishido et al., J.S.C.I. Japan, 45:436 (1942)). Betulin has also been isolated from rosehips (Zimmermann, Helv. Chim. Acta, 27:332 (1944)) and from the seeds of Zizyphus vulgaris Lamarck var. spinosus Bunge (Rhamnaceae) (Kawaguti et al., J. Pharm. Soc. Japan, 60:343 (1940)). Ruhemann et al., Brennstoff-Ch., 13:341 (1932) discloses the presence of betulin, allobetulin, and an “oxyallobetulin” in the saponifiable portion of a benzene-alcohol extract of mid-German brown coal. In addition, the following group of lupon-row derivatives from the birch cortex extract have been identified: (a) betulinol, (b) betulinic acid, (c) betulin aldehyde, (d) betulonic acid, and (e) betulon aldehyde (Rimpler et al., Arch. Pharm. Und. Ber. Dtsh. Ppharmaz Jes, 299:422-428 (1995); Lindgren et al., Acta Chem., 20:720 (1966); and Jaaskelainen, P. Papperi Ja Puu-Papper Och Tra., 63:599-603 (1989)).
Birch tree cortex-extracted betulinol was first mentioned as an antiseptic in 1899. Subsequently, compounds singled out from extracts of Hyptis emory and Alnus oregonu, identified as pentacyclic styrenes and their derivatives, were shown to inhibit carcinosarcoma growth (Sheth et al., J. Pharm. Sci., 61:1819 (1972) and Sheth et al., J. Pharm. Sci., 62:139-140 (1973)). It has been suggested that betulinic acid is the main anti-tumor agent in the mixture of terpenoids (Tomas et al., Planta Medicina, 54:266-267 (1988) and Ahmat et al., J. Indian Chem. Soc., 61:92-93 (1964)). In particular betulinic acid showed cytotoxic activity against carcinoma cell line CO-115 of the large intestine (LD 50=0.375 mg/ml) (Ukkonen et al., Birch bark extractive kemia kemi, 6:217 (1979)).
The use of chemotherapeutic agents in the treatment of a variety of cancers has become a well established part of cancer treatment regimens, especially where the disease has progressed to an advanced stage. However, these chemotherapeutic agents act not only on malignant cells but have adverse effects on non-target cells as well, particularly on the rapidly proliferating cells of the gastrointestinal tract and bone marrow. When employed in the high concentrations frequently required to be effective in killing cancer cells, these cytotoxic drugs give rise to undesirable and frequently severe side effects. Although the concept of site-directed chemotherapy is quite old, only a small number of anti-neoplastic drugs and toxins have been successfully coupled to monoclonal and polyclonal antibodies.
Therefore, a need continues to exist for chemotherapeutic agents and, in particular, for site-directed chemotherapeutic agents. The present invention is directed to meeting this need.